Treat untracked phabricator repos the same as deleted.

Summary:
We don't necessarily want to delete old phabricator repos, since there
might still be urls pointing to into the repo (e.g. an audit, which is
keyed by repo-callsign).  There's no reason to make those urls stop
working.  Instead, we can just 'untrack' the repo, which means that it
just sits there and otherwise phabricator ignores it: audits don't
work, differential revisions aren't updated, etc.  That's kinda like
being deleted.

This change makes it so we don't tell you to delete a phabricator repo
if it's untracked.

Test Plan:
I went to http://phabricator.khanacademy.org/repository and made all
the kiln Code/Mobile-Apps and Code/Private hg repos untracked (the git
repos are the canonical ones now), and then ran
   ./update_phabricator_repositories.py -n /var/tmp/
and it said:
   [...]
   * Existing UNTRACKED phabricator repos:
   https://khanacademy.kilnhg.com/Code/Mobile-Apps/Group/android
   https://khanacademy.kilnhg.com/Code/Mobile-Apps/Group/hp-touchpad
   https://khanacademy.kilnhg.com/Code/Mobile-Apps/Group/iPad
   https://khanacademy.kilnhg.com/Code/Mobile-Apps/Group/iPad-exercises
   https://khanacademy.kilnhg.com/Code/Mobile-Apps/Group/iPad-old
   https://khanacademy.kilnhg.com/Code/Private/Group/assistly
   https://khanacademy.kilnhg.com/Code/Private/Group/backup-and-analytics
   https://khanacademy.kilnhg.com/Code/Private/Group/office-tools
   https://khanacademy.kilnhg.com/Code/Private/Group/profanity-tools
   https://khanacademy.kilnhg.com/Code/Private/Review/snippets
   https://khanacademy.kilnhg.com/Code/Private/Review/test

   Adding 0 new repositories to phabricator
   Removing 0 deleted repositories from phabricator

Yay, no repositories to delete!

Reviewers: chris, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D4159